Liquid flow meters have application in many fields for measuring the flow of liquid through conduits. The true mass flow of liquid through a conduit depends upon the temperature of the liquid, since the liquid expands with temperature. The temperature of the liquid may vary because of changes in ambient temperature or because of artificial heating to improve efficiency of pumping in the case of liquids which have high viscosity at low temperatures. Accordingly, there is a requirement to provide compensation in the output of a flow meter in accordance with the temperature of the liquid.
A temperature-dependent mechanical output has been achieved hitherto by the use of a fluid-filled capsule immersed in the liquid of which the flow is being measured. The capsule is coupled to a remote bellows unit by capillary tubing and the variation of expansion of the fluid by virtue of the temperature of the liquid changes the pressure on the bellows and thus effects a mechanical movement. However, ambient temperature changes at the bellows unit and the capillary tubing also produce mechanical movement which would give rise to an error. Compensation for such ambient temperature changes of the bellows unit and capillary tubing has been effected by providing a second sealed bellows arrangement in tandem with the first and assembled in such a way that changes in ambient temperature are compensated for. Such a prior arrangement is convenient in that the small diameter capillary tubing makes it possible to mount the capsule in a convenient position and allows easy coupling to a flow indicator compensation arrangement. However, the provision of a second bellows arrangement for the correction of ambient temperature effects is unduly complex. The present invention seeks to provide an improved temperature compensating arrangement.